Antenna matching network utilizing an adjustable high-power inductor

ABSTRACT

An adjustable high-power inductor comprising a toroidal inductance coil having a stationary two-section ferromagnetic core whose confronting ends form a pair of predetermined air-gaps therein. The equivalent reluctance of the magnetic flux path, and thus the inductance, is varied by rotating a disc through the air-gaps and about an axis parallel to the magnetic flux lines generated within the air-gaps. The disc carries a pair of diametrically opposed ferromagnetic members, each having a tapered profile. A reversible motor controls the angular position of the disc, and the angular displacement of the disc determines the equivalent reluctance of the magnetic flux path.

United States Patent 1191 Sheffield 1 51 July 3, 1973 ANTENNA MATCHING NETWORK UTILIZING AN ADJUSTABLE HIGH-POWER INDUCTOR Berthold Sheffield, Belle Mead, NJ.

Assignee: RCA Corporation, New York, NY. Filed: Dec. 22, 1971 Appl. No.: 210,770

Inventor:

References Cited UNITED STATES PATENTS 6/1970 Dawson 8! al. 336/135 x 11/1967 Bruene ass/17 x 10/1969 Ainsworth 333/l7 Primary Examiner-Paul L. Gensler Attorney-Edward J. Norton [57] ABSTRACT An adjustable high-power inductor comprising a toroidal inductance coil having a stationary two-section ferromagnetic core whose confronting ends form a pair of predetermined air-gaps therein. The equivalent reluctance of the magnetic flux path, and thus the inductance, is varied by rotating a disc through the air-gaps and about an axis parallel to the magnetic flux lines generated within the air-gaps. The disc carries a pair of diametrically opposed ferromagnetic members, each having a tapered profile. A reversible motor controls the angular position of the disc, and the angular dis placement of the disc determines the equivalent reluctance of the magnetic flux path.

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ANTENNA MATCHING NETWORK UTILIZING AN ADJUSTABLE HIGH-POWER INDUCTOR BACKGROUND OF THE INVENTION This invention relates to adjustable inductances and more particularly to high-power inductors, the inductance of which can be varied.

High-power inductors are used in many industrial applications. For instance, in a VLF (very low frequency) high-power transmitter, which may operate in the 300B, to 30KB frequency range, inductive elements are used in the tank circuit as part of the resonant circuit; and in the antenna matching circuit in which the inductor is adjusted to cancel antenna reactance so that maximum power can be transferred to the antenna. In these applications an adjustable inductance is preferred to an adjustable capacitance for tuning purposes due to the unrealistic physical size requirements of a capacitor at these low frequencies. However, the inductors are required to carry large currents because of the high power levels and to exhibit relatively large values of inductance because of the low operating frequencies. Therefore, for efficient operation the inductors must be capable of accurate and continuous adjustment with full power applied. Further, the adjustment means should have a fast response time; permit adjustment over a substantial percentage change in inductance; and consume relatively low power.

Basically, an induction coil mounted on a magnetic core with a winding having a suitable conductor crosssection meets the requirements for power handling capability and large values of inductance. In the prior art, these inductors have used tapped windings with mechanical or electronic switching means to select the desired number of turns and therefore the inductance. However, since the tap points represent discrete inductance values, fine adjustment of inductance is not readily achieved.

Another known method for adjusting such an inductor is to vary the reluctance of the magnetic circuit within the core by providing an adjustable air-gap therein whose dimension may be varied. In the prior art the air-gap dimension has been varied by further separating or closing the air-gap; or by positioning a magnetic member away from or into greater proximity to the air-gap. However, in high-power applications, under full current load conditions, the mechanical forces required to separate the gap precludes efficient, accurate and fast-response adjustment of the inductor; and, the mechanical vibrations present in these prior art adjustable inductors when used in high-power applications, such as VLF transmitters, prevent fine adjustment of the inductor. Further, the limited adjustment range of prior art adjustable inductors in not acceptable in all applications.

These problems and disadvantages are overcome in the present invention by providing a continuous and efficient means for accurately varying the inductance of a high-power inductor over a wide range of values.

SUMMARY OF THE INVENTION The invention is characterized by the provision of an adjustable inductor that is particularly suited for use as an inductive element in high-power VLF transmitters. The adjustable inductor comprises a winding on a magnetic core with confronting ends that define an air-gap. A movable member of magnetic material adapted for movement within the confronting ends together with the magnetic core completes a magnetic flux path. Means are provided for moving the movable member through the confronting ends. The adjustable inductor includes adjustable flux path means defined by the movable member with the cross-sectional area of the movable member along the direction of its movement varying between a minimum and maximum. Consequently, the equivalent reluctance of the magnetic flux path varies according to the position of the movable member.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an adjustable toroidal core inductance coil constructed in accordance with the principle of the present invention;

FIG. 2 is a perspective view of the movable disc employed in the inductance coil of FIG. 1;

FIG. 3 is a cross-sectional view of the movable disc of FIG. 2 taken along the line 3-3 of FIG. 2; and

FIG. 4 is a combined schematic and block diagram of an antenna matching network illustrating one manner in which the adjustable inductor shown in FIG. 1 may be used.

DETAILED DESCRIPTION FIG. 1 shows at 10 an adjustable toroidal inductance coil including two C-shaped sections ofa magnetic core material. Core sections 12a and 12b are secured to a base 13 by supporting brackets 14 and 15 in such a manner that the confronting ends form two air-gaps. The cores may be made of any magnetic material including ferromagnetic materials such as, for example, iron. The brackets 14 and 15 are made preferably of a nonmagnetic material. Mounted on the cores are multilayer windings 16a, 16b, 16c and 16d. The windings are connected in series wth their free ends 17 and 18 brought out for connection of an external circuit (not shown). -Core sections 12a and 12b are provided with holes bored through the magnetic material on a diameter of the toroid which is perpendicular to the confronting ends of the core sections and preferably at the midpoint of the vertical dimension so as to maintain symmetry. Sleeves 20a and 20b are inserted into the holes and may extend from the outer periphery of the toroid to the inner periphery. A movable disc 22 is mounted on and fastened to a shaft 24 by a coupling flange 26. Shaft 24 is inserted through sleeves 20a and 20b in such a manner as to permit disc 22 to rotate through the airgaps formed by the confronting ends of core sections 12a and 12b. Shaft 24 is coupled to the output shaft 27 of a reversible motor 28 by coupling 30. The control leads 32a and 32b of motor 28 are brought out for connection to an external circuit (not shown). The reversible motor 28 may be of any conventional type but is preferably of a type that develops full power rapidly when an electrical control signal is applied thereto and remains locked when the control signal is removed.

Referring now to FIG. 2, there is shown a perspective view of the movable disc 22 employed in the apparatus of FIG. 1. The disc 22 is made preferably of a nonmagnetic material in which magnetic members 34a and 34b are embedded. The members 34a and 34b are embedded at diametrically opposite ends of disc 22. The cross-sectional areas of members 34a and 34b, perpendicular to the face of the disc, are shaped in such a manner that the amount of magnetic material present in the air-gaps when the disc 22 is rotated through the air-gaps varies according to the angular displacement of disc 22. Since the magnetic members are carried by the disc in an opposed relation, amechanical balance which facilitates rotation of the disc is thereby achieved.

As shown more clearly in FIG. 3, the dimension t of disc 22 is preferably just slightly less than the dimension of the fixed air-gaps so as to permit free rotation of disc 22 in the air-gaps but to maintain intimate contact with the confronting ends of core sections 12a and 12b, thereby substantially precluding motion perpendicular to the face of the disc. Further, disc 22 is preferably coated with a suitable nonmagnetic material, such as for example, an epoxy resin, having a thickness K, so as to provide a minimum effective air-gap between the confronting ends of core sections 12a and I2b regardless of the angular position of disc 22. The minimum effective air-gap provided in this manner prevents magnetic saturation of the core material even when the coil windings are carrying relatively large currents.

As shown at 34 in FIG. 3, the individual magnetic members 34a and 34b of FIG. 2 comprise stacked sheets of magentic material in order to form magnetic laminations. In this manner eddy current losses in the magnetic members 34a and 34b are reduced substantially. This construction has the advantage that heating losses in the members are reduced and that eddy current braking or drag on disc 22, as it rotates through the magnetic field established in the air-gaps, is also reduced. It should be noted, however, that these eddy current losses may also be reduced by employing a nonconducting magnetic material having a conductivity significantly less than the conductivity of iron, such as a ferrite, in the construction of magnetic members 34a and 34b.

Each magnetic member 34a and 34b may form a semi-annular ring approaching 180. However, by providing a sufficient segment of nonmagnetic material along the outer annulus of disc 22, a maximum effective air-gap can be provided between the confronting ends of core sections 12a and 12b when disc 22 is in the corresponding angular position. It should now be apparent that by rotating disc 22, the amount of magnetic material in the fixed air-gaps may be varied between a minimum amount corresponding to the maximum effective air-gap and a maximum amount which corresponds to the minimum effective air-gap.

Core sections 12a and 12b, and disc 22 form a magnetic flux path circuit whose equivalent reluctance is varied according to the angular position of disc 22. The reluctance of this magnetic flux path decreases as the amount of magnetic material in the fixed air-gaps increases. Since the inductance ofa coil wound on a magnetic core is inversely proportional to the reluctance of the flux path within the core, the inductance of the toroidal coil, shown in the drawings, varies in accordance with the angular position of disc 22. It should be apparent that the above-described variation of inductance is achieved without the use of moving contacts or external switching circuits coupled to the coil windings.

With the construction shown in the drawings, it has been found that a maximum to minimum inductance ratio of 1.64 to I can be readily secured by the described arrangement. Further, with careful construction practice with respect to providing a minimum effective air-gap, ratio of at least 3 to 1 should readily be obtained. It has also been found that with the shown construction the force required to rotate the disc with the inductor energized is substantially less than the force required to physically separate the confronting ends of the core sections in a comparable test. This reduction is explained by the relatively small change of inductance per degree of angular rotation of the movable member. Additionally, since the core sections are fixed relative to each other, no external force is required to maintain the fixed air-gap therebetween or to move the mass of the core sections relative to each other. Moreover, once the inductor, in accordance with the present invention, is adjusted, no further power is required to maintain that setting.

Although the movable member described in conjunction wth the present invention is preferably a flat disc, other geometric configurations can produce a like result. For example, disc 22 may be constructed in a saucer shape having a thickness which tapers to a point at the edge of the disc. Further, the magnetic members 34a and 34b may be constructed in any one of a number of suitable geometric shapes, for example, the magnetic member may exhibit a tapered edge cross-section. In any case, since it is desirable to provide a minimum effective air-gap, the shape of the confronting ends of the magnetic core sections should exhibit a corresponding contour.

Referring now to FIG. 4, there is shown a block diagram of an antenna matching network system employing the adjustable inductor shown in FIG. 1. An input terminal 102 of antenna matching network is connected to a source 104 of VLF energy. An output terminal 106 of matching network 100 is connected to an antenna load 108. The input terminal 102 of network 100 is also connected to one terminal of adjustable inductor 110. The other terminal of inductor 110 is cou pled to the output terminal T06 of network 300 by way of capacitor 112. Capacitor 112 may comprise two or more fixed capacitors adapted for series/parallel connection wherein the equivalent capacitance of capacitor 112 is controlled by external switching means. Inductor 110 includes an adjustment means 114 which is depicted schematically as an arrow. A reversible motor 116 is shown coupled to adjustment means 114 by way of output means 118. The output voltage waveform of source 104 is sampled by way of lead 120 which in turn is coupled to the input of a voltage waveform detector 121. The output of detector 121 provides a first input to a phase comparator 122. The current output waveform of source 104 is sampled by way of lead 124 which is in turn coupled to the input of current waveform detector 125. The output of comparator 122 is coupled to the input of a control circuit 126. The output of control circuit 126 is coupled to the control input of reversible motor 116.

The purpose of antenna matching network 100 is to maximize power transfer from source 104 to the antenna load 108 by cancelling the reactive component of the impedance of load 108. Maximum power transfer occurswhen the impedance of antenna load 108 is purely resistive. When the reactance of antenna matching network 106 is of opposite sign and equal to the reactive component of the impedance of antenna load 108, the resulting impedance of antenna load 108 is purely resistive. Further, when the resulting impedance of antenna load MP8 is purely resistive, the voltage and current waveforms at the output of source 104 are in phase and the output of phase comparator 122 is zero.

The output of control 126 controls reversible motor 116 in response to the output of phase comparator 122. When the detected voltage waveform leads the detected current waveform, the output of phase comparator 122 causes the output 118 of motor 116 to operate in a first direction. Similarly, when the current waveform leads the voltage waveform, the output of phase comparator 122 causes the output means 118 of motor 116 to operate in a direction opposite that of the first direction. Finally, when the voltage and current waveforms are in phase, the output of phase comparator 122 is zero and the output 118 of motor 116 remains locked in a fixed position.

In practice the current waveform may be detected by a conventional current transformer or by providing a small resistive impedance relative to the impedance of antenna load 108 in series with the input or output of matching network 100. The voltage waveform may be detected in any conventional manner such as by detecting the voltage developed across a relatively high impedance which may be bridged across the output of source 104.

It should be noted that by selecting a suitable value for capacitor 112, the resulting impedance of serially connected inductor 110 and capacitor 112 can be either a capacitive or inductive reactance. That is, even though the variable component of antenna matching network 100 is an adjustable inductor, the terminal characteristics of matching network 100 can be selected so as to appear as an adjustable capacitor. That is of particular utility as the reactive component of the impedance of VLF antennas is typically inductive.

What has been shown then is a high-power adjustable inductor that is suited particularly for use as an inductive element in the output circuit of VLF transmitters. ln these applications the adjustable inductor in accordance with the present invention provides a relatively high inductance which may be continuously and efficiently varied over a wide range of values while the inductor is carrying large currents. It should be apparent, however, that the present invention is not limited to VLF applications but will find utility in other applications where an adjustable high-power inductor is required.

What is claimed is:

-1. An adjustable inductor, comprising, in combination:

a magnetic core comprising two generally C-shaped core sections forming a toroidal core having two pairs of fixed confronting ends defining two air gaps substantially along a diameter of said toroidal core;

a movable member of magnetic material adapted for movement within said confronting ends, said 'movable member comprising a nonmagnetic disc having a pair of diametrically opposed magnetic members embedded therein, said disc being mounted on a fixed axis substantially perpendicular to said diameter and adapted for rotation about said axis;

said magnetic core with said movable member providing a magnetic flux path;

means for moving said movable member through said confronting ends;

adjustable flux path means defined by said movable member with the cross-sectional area of said movable member along the direction of its movement varying between a minimum and maximum value wherein the equivalent reluctance of said magnetic flux path varies according to the position of said movable member; and

a winding on said magnetic core with terminals adapted for coupling to an external electrical circuit.

2. The adjustable inductor according to claim 1 wherein said opposed magnetic members each comprise a plurality of stacked sheets of magnetic material forming a laminated magnetic member.

3. The adjustable inductor according to claim 1 wherein said means for moving said member includes a motor having an output means adapted for rotational motion in response to the application of a control signal to said motor and means for translating the rotational motion of said output means to rotational movement of said disc.

4. The adjustable inductor according to claim 3 wherein said motor comprises a-reversible motor.

5. The adjustable inductor according to claim 1 wherein said disc includes means for providing minimum effective air-gap within said confronting ends.

6. The adjustable inductor according to claim 5 wherein said opposed magnetic members each comprise a magnetic material having a conductivity significantly less than the conductivity of iron.

7. An antenna matching network including an adjustable reactance tuner having a first terminal adapted for connection to a source of VLF signals and a second terminal adapted for coupling to an antenna, said adjustable reactance tuner including an adjustable inductor comprising:

a magnetic core comprising two generally C-shaped core sections forming a toroidal core having two pairs of fixed confronting ends defining two air gaps substantially along a diameter of said toroidal core;

a movable member of magnetic material adapted for movement within said confronting ends, said movable member comprising a nonmagnetic disc having a pair of diametrically opposed magnetic members embedded therein, said disc being mounted on a fixed axis substantially perpendicular to said diameter and adapted for rotation about said axis;

said magnetic core with said movable member providing a'magnetic flux path;

means for moving said movable member through said confronting ends;

adjustable flux path means defined by said movable member with the cross-sectional area of said member along the direction of its movement varying between a minimum and maximum value, wherein the equivalent reluctance of said magnetic flux path varies according to the position of said movable member; and

a winding on said magnetic core with terminals adapted for coupling between said first and second terminals.

8. The antenna matching network according to claim 7 wherein said means for moving said member includes means coupled to said disc and adapted for rotating said disc in response to phase variations between voltage and current waveforms at said first terminal.

9. The antenna matching network according to claim 8 wherein said adjustable reactance tuner includes a capacitor serially coupled with said adjustable inductor.

10. The antenna matching network according to claim 9 wherein said capacitor exhibits a capacitive reactance whose relative magnitude exceeds the maximum inductive reactance of said adjustable inductor at a given frequency of said VLF signals.

11. An adjustable reactance device comprising in combination:

a magnetic core comprising two generally c-shaped core sections forming a toroidal core having two pairs of fixed confronting ends defining an two air gaps substantially along a diameter of said toroidal core;

a movable member of magnetic material adapted for movement within said confronting ends, said movable member comprising a nonmagnetic disc having a pair of diametrically opposed magnetic members embedded therein, said disc being mounted on a fixed axis substantially perpendicular to said diameter and adapted for rotation about said axis;

said magnetic core with said movable member providing a magnetic flux path;

means for moving said movable member through said confronting ends;

adjustable flux path means defined by said movable member with the cross-sectional area of said movable member along the direction of its movement varying between a minimum and maximum value wherein the equivalent reluctance of said magnetic flux path varies according to the position of said movable member;

a winding on said magnetic core;

at least one fixed capacitor serially coupled with said winding, wherein the capacitive reactance of said capacitor is added to the relative inductive reactance of said winding; and

first and second output terminals, said first output terminal being coupled to one end of said winding and said'second output terminal being coupled to one end of said fixed capacitor, said output terminals being adapted for coupling to an external circuit, whereby the resultant reactance at said output terminals varies according to the position of said movable member.

12. The adjustable reactance device according to claim 11 wherein said means for moving said member includes a motor having an output means adapted for rotational motion in response to the application of a control signal to said motor and means for translating the rotational motion of said output means to rotational movement of said disc. 

1. An adjustable inductor, comprising, in combination: a magnetic core comprising two generally C-shaped core sections forming a toroidal core having two pairs of fixed confronting ends defining two air gaps substantially along a diameter of said toroidal core; a movable member of magnetic material adapted for movement within said confronting ends, said movable member comprising a nonmagnetic disc having a pair of diametrically opposed magnetic members embedded therein, said disc being mounted on a fixed axis substantially perpendicular to said diameter and adapted for rotation about said axis; said magnetic core with said movable member providing a magnetic flux path; means for moving said movable member through said confronting ends; adjustable flux path means defined by said movable member with the cross-sectional area of said movable member along the direction of its movement varying between a minimum and maximum value wherein the equivalent reluctance of said magnetic flux path varies according to the position of said movable member; and a winding on said magnetic core with terminals adapted for coupling to an external electrical circuit.
 2. The adjustable inductor according to claim 1 wherein said opposed magnetic members each comprise a plurality of stacked sheets of magnetic material forming a laminated magnetic member.
 3. The adjustable inductor according to claim 1 wherein said means for moving said member includes a motor having an output means adapted for rotational motion in response to the application of a control signal to said motor and means for translating the rotational motion of said output means to rotational movement of said disc.
 4. The adjustable inductor according to claim 3 wherein said motor comprises a reversible motor.
 5. The adjustable inductor according to claim 1 wherein said disc includes means for providing minimum effective air-gap within said confronting ends.
 6. The adjustable inductor according to claim 5 wherein said opposed magnetic members each comprise a magnetic material having a conductivity significantly less than the conductivity of iron.
 7. An antenna matching network including an adjustable reactance tuner having a first terminal adapted for connection to a source of VLF signals and a second terminal adapted for coupling to an antenna, said adjustable reactance tuner including an adjustable inductor comprising: a magnetic core comprising two generally C-shaped core sections forming a toroidal core having two pairs of fixed confronting ends defining two air gaps substantially along a diameter of said toroiDal core; a movable member of magnetic material adapted for movement within said confronting ends, said movable member comprising a nonmagnetic disc having a pair of diametrically opposed magnetic members embedded therein, said disc being mounted on a fixed axis substantially perpendicular to said diameter and adapted for rotation about said axis; said magnetic core with said movable member providing a magnetic flux path; means for moving said movable member through said confronting ends; adjustable flux path means defined by said movable member with the cross-sectional area of said member along the direction of its movement varying between a minimum and maximum value, wherein the equivalent reluctance of said magnetic flux path varies according to the position of said movable member; and a winding on said magnetic core with terminals adapted for coupling between said first and second terminals.
 8. The antenna matching network according to claim 7 wherein said means for moving said member includes means coupled to said disc and adapted for rotating said disc in response to phase variations between voltage and current waveforms at said first terminal.
 9. The antenna matching network according to claim 8 wherein said adjustable reactance tuner includes a capacitor serially coupled with said adjustable inductor.
 10. The antenna matching network according to claim 9 wherein said capacitor exhibits a capacitive reactance whose relative magnitude exceeds the maximum inductive reactance of said adjustable inductor at a given frequency of said VLF signals.
 11. An adjustable reactance device comprising in combination: a magnetic core comprising two generally C-shaped core sections forming a toroidal core having two pairs of fixed confronting ends defining an two air gaps substantially along a diameter of said toroidal core; a movable member of magnetic material adapted for movement within said confronting ends, said movable member comprising a nonmagnetic disc having a pair of diametrically opposed magnetic members embedded therein, said disc being mounted on a fixed axis substantially perpendicular to said diameter and adapted for rotation about said axis; said magnetic core with said movable member providing a magnetic flux path; means for moving said movable member through said confronting ends; adjustable flux path means defined by said movable member with the cross-sectional area of said movable member along the direction of its movement varying between a minimum and maximum value wherein the equivalent reluctance of said magnetic flux path varies according to the position of said movable member; a winding on said magnetic core; at least one fixed capacitor serially coupled with said winding, wherein the capacitive reactance of said capacitor is added to the relative inductive reactance of said winding; and first and second output terminals, said first output terminal being coupled to one end of said winding and said second output terminal being coupled to one end of said fixed capacitor, said output terminals being adapted for coupling to an external circuit, whereby the resultant reactance at said output terminals varies according to the position of said movable member.
 12. The adjustable reactance device according to claim 11 wherein said means for moving said member includes a motor having an output means adapted for rotational motion in response to the application of a control signal to said motor and means for translating the rotational motion of said output means to rotational movement of said disc. 